A Royal Military Police officer has been shot dead by a British soldier in a "friendly fire" accident in southern Afghanistan, according to defence sources.

The serviceman, the 104th member of UK forces to be killed in the country this year, was on foot patrol in the Sangin area of Helmand province when he was shot on Sunday evening. Officials ruled out the possibility that he was shot by Afghans, either deliberately or mistakenly, or by American troops.

An investigation is under way but the cause of death will not be officially revealed until the inquest which may not be held for a year.

News of the accidental shooting came as the Guardian reveals for the first time that US special forces have conducted several clandestine raids into Pakistan as part of a secret war being waged against the Taliban and al-Qaida in the areas bordering Afghanistan.

The disclosure is likely to prove inflammatory in Pakistan where there was outrage over the only previously admitted US raid in 2008. A former Nato officer told the Guardian that Islamabad had not been informed about three other incursions.

Meanwhile the MoD confirmed it was "a possibility" that the military policeman's death was caused by friendly fire from a British soldier. "The incident is being investigated in Afghanistan but no firm conclusion will be reached until after the coroner's inquest," the ministry said.

Military policemen carry out many patrols with British and Afghan troops in Helmand. They also investigate incidents involving shootings and captured insurgents. Defence sources last night indicated that the British soldier who shot the military policeman was not in any way involved in controversy or believed to have run amok.

The death takes to 241 the total number British troops killed in Afghanistan since late 2001. It is the sixth death among members of the military police this year, five of whom have been killed in the past two months. Two military policemen were among five British servicemen killed last month when an Afghan policeman being trained by UK forces opened fire at a military compound.

This year has been the bloodiest for British forces during the eight-year Afghan campaign, with the majority killed by sophisticated roadside bombs planted by insurgent groups.

Military commanders and ministers are braced for more of the same next year as international forces make a major push against Taliban-allied forces, boosted by a "surge" of 30,000 more US troops pledged earlier this month by Barack Obama. Britain will add another 500 troops to its 9,000-strong contingent, part of a 7,000 troop increase promised from other Nato countries.

Last week, the defence secretary, Bob Ainsworth, said the government would buy 22 new Chinook helicopters and other equipment destined for Afghanistan, with an RAF base being closed and some jobs cut to fund the reallocation of resources.

The Afghan surge is intended to run alongside a concerted effort to train and equip the country's own defence forces so they can take over greater responsibilities. Afghanistan's defence minister said yesterday that the country expected to receive more than 150 aircraft from western allies over the next six years, quadrupling the size of its air force.